Images on electronic displays are formed by an array of small picture elements known as pixels. In electronic color displays, these pixels comprise three color elements that produce the primary colors red, blue and green for matching any other color. Usually arranged as squares or rectangles, the pixel array can be characterized by pixel pitch, P, a quantity that measures the density of pixels per unit distance. A typical cathode-ray tube display has a pixel pitch of 0.3 mm. Typical small computer screens have a pixel array width-to-height ratio of 4 to 3. Pixel arrays in electronic displays are typically disposed in 640.times.480 (VGA), 800.times.600 (SVGA) or 1024.times.768 (XGA) configurations.
Virtually all commercial display products are manufactured as a single monolithic pixel array with a constant pixel pitch across the entire face of the display. While this produces continuous images across the entire display, at the same time, it limits the sizes of the pixel arrays to what can be manufactured as a single display unit given the yield of the fabrication processes and assembly techniques employed. For example, cathode ray tube (CRT) displays are limited to a screen diagonal of about 35" and active matrix liquid crystal displays (AMLCD) to about 15".
In principle, larger displays comprising a plurality of adjacent display units, arranged as tiles like a bathroom floor, can be used to overcome the size limitations of manufacturing and assembly processes. However, this has proven extraordinarily difficult. The reason lies in the seams generated between the tiles from the manufacturing and assembly processes. Because of edges in the display tiles, assembly, alignment, and brightness and color non-uniformity on adjacent tiles, the seams not only become visible, but produce image segmentation, brightness offsets, and color errors that are unacceptable to the human eye. A well documented but failed effort to manufacture tiled displays is given by Magnascreen Corporation, a Pittsburgh based company that ceased operations in the mid 1990's.sup.[1]. Despite these difficulties it is desirable to develop methods for tiling displays in order to overcome manufacturing and assembly limitations. The worldwide display market is expected to be on the order $40 B by the year 2000; greatest growth potential is in large displays.
The pixel pitch in electronic displays is set so that at the minimum viewing distance the human eye cannot resolve individual pixels. This limit is about 0.5 arc-minutes for typical display viewing conditions. For example, with a pixel pitch of P=0.3 mm, the minimum viewing distance becomes about three feet. Therefore, the width of any seams that are placed between the tiles in assembled displays must generally be less than the pixel pitch in order for the seam to become imperceptible. This is a very severe requirement that rules out most display technologies. For example, the CRT requires the pixel array to be enclosed in a vacuum vessel made of glass that is much thicker than the pixel pitch.
Assemblies of displays having wide display units or unusually large pixel pitches between adjacent display units are generally called multi-screen displays and do not possess imperceptible seams. Rather, the seams are usually deliberately covered by black stripes in order to make them less disturbing. Examples of such a multi-screen display apparatus have been described by Watanabe et al. from Mitsubishi .sup.[2] and Someya from Hitachi .sup.[3]. In both of these cases, color corrections are applied to larger regions of independent displays with very large pixel gaps between the display units. Therefore, image correction methods developed for such display assemblies, whether applies to geometric patterns, brightness, or color, have been designed to apply to pixels always separated by a large spacing at least as wide as the seam. Therefore the application of such methods to tiled displays with closely spaced pixels (uniform pixel pitch) will not produce imperceptible seams. Human visual perception has been found to be very sensitive to minute discontinuities in images, whether of a geometrical, photometric or calorimetric nature. This invention describes methods that apply to tiled displays with uniform pixel pitches over the entire display surface, including across seams, and therefore has to deal with displayed images to the most minute discontinuities.
Flat-panel displays (FPDs) are good candidates for making tiled displays, because the edges of the display elements (tiles) can potentially be made small enough. FPDs include liquid crystal displays (LCDs), active matrix LCDs (AMLCDs), plasma displays (PDs), field emission displays (FEDs), electroluminescent displays (ELDs) and digital mirror displays (DMDs), all of which depend on the microfabrication of the key components carrying the pixel patterns. AMLCD is a technology currently favored by the industry and it has a large market share, in particular in notebook computers. For purposes of clarity, the term "LCD" is used herein, but is intended to describe all types of flat-panel displays.
From a practical point of view, the yield of micro-fabrication is unacceptable for large size displays, due to the fact that yield generally falls exponentially with area and an unacceptable number of fabricated pieces must be rejected. The present inventors, therefore, have determined that smaller pixel arrays (tiles) can be microfabricated and, after appropriate selection, assembled together to form a larger display comprising an array of tiles. However, past attempts have led to visible seams due, in large part, to the space required by tile edges and assembly into the full displays. Hence, pixel spacings across seams have been much larger than those required of monolithic displays. This is essentially the reason for the fact that very few attempts have been made to achieve large, color, "seamless", tiled displays.
In co-pending U.S. patent applications, Ser. Nos. 08/593,759 and 08/571,208, filed on Jan. 29, 1996 and Dec. 12, 1995, respectively, a method of constructing a seamless, tiled, flat-panel display is illustrated. The teachings of these companion applications are meant to be incorporated herein by way of reference.
The electronic circuitry built into or used with conventional monolithic, non-tiled display has two functions:
(1) transform the incoming electronic representation or video signal of the image to be displayed into a format compatible with the display device, and send this tranformed signal continuously and in real time to the display device; and
(2) provide a set-up and adjustment capability to the display. Brightness, contrast, threshold, tint, white point and reference levels are examples. Some of these adjustments can be set by both the viewer and the display manufacturer; others are deliberately made inaccessible to the viewer. To the display manufacturer, these adjustments allow reasonable manufacturing tolerances for the components of the display. They also allow for variations across the viewing area of the display unit that occur, at least in part, due to an inherent non-uniformity of manufacturing processes. As the result these adjustments present a more acceptable picture quality to the viewer. In addition, these adjustments allow some picture quality attributes to be changed, in order to suit individual viewing preferences and viewing conditions.
The extension of the first function above, transformation of the video data from a monolithic display to a tiled display, is straightforward to a person skilled in the art of displays. This invention describes methods of extending the second function, display adjustments to a tiled display, in such a way that the displayed picture quality is equivalent to a non-tiled display. Note that this is a much more difficult undertaking than adjusting multi-screen display apparatuses as discussed above. Alternative methods for improving the picture quality on a tiled display and making it visually comparable or superior to that of a monolithic display can be based on new techniques for the aforementioned function (1) and combinations of functions (1) and (2).
The present invention provides unique circuitry and a display tile assembly for achieving color purity in a "seamless" tiled display, comprising a tiled mosaic of individual LCDS. In any tiled displays, that are commercially acceptable, color purity has to be uniform within each tile. That is, there should be no apparent differences in brightness or color between tiles over the entire range of input video signals to be rendered. Note that this is a more stringent requirement on tiled displays than on monolithic displays, because the human visual system accepts smooth non-uniformities as large as 10-20%, while abrupt changes have to be controlled to about 1%.
The optical performance of a display can be characterized by parameters that describe the voltage input to pixels and the resulting transmission of the elements. For example, AMLCDs have threshold voltages V.sub.TH and V.sub.Dmux for maximum and minimum transmission, T.sub.max and T.sub.min (see FIG. 1aa). The pixel optical gain, V.sub.SL, can be described as the slope of the transmission-voltage curve. Color coordinates may also vary. A similar set of parameters can be identified for other types of flat-panel displays.
In the extension from conventional monolithic displays to tiled displays, additional parameters related to the quality of the display near the edge of each tile can be identified, for example, due to the filling of the liquid crystal material or the thickness of the cell gap. Further abrupt variations arise from the butting of adjacent tiles, each with smooth variations typical of monolithic displays. Other optical components of the display may also vary.
Color purity is defined as the condition of uniform saturation of primary colors over the screen. There are several sources of inter-tile color differences, including differences in the color coordinates between tiles, threshold and transmission voltages in the pixels adjacent the seams, color filter variations from tile to tile, etc.
The inventors realize that individual tiles can provide acceptable color purity; therefore, a standard LCD controller/driver chip set, as is commercially available for example, from Toshiba or Hitachi, can be integrated into a circuit that is used to achieve total color purity throughout a tiled LCD display. However, the exclusive use of conventional controller and driver circuits will not render a seamless display. Therefore, further circuits are added in order to achieve color corrections to a level that appears uniform to the average human observer.